Negative and Positive bias temperature instability (NBTI and PBTI) and time dependent dielectric breakdown (TDDB) are important reliability issues in MOSFETs. NBTI and PBTI are collectively referred to below as bias temperature instability (BTI).
NBTI affects p-channel MOS devices, which usually operate with negative gate-to-source voltage, as well as nMOS transistors when biased in the accumulation regime with a negative bias applied to the gate. NBTI increases the threshold voltage VT and decreases drain current and transconductance. The NBTI degradation can be characterized by the fractional reduction in drive current for a given drive voltage or by the increase in leakage current.
A substantial portion of the degradation due to NBTI is recovered over time during use. PBTI affects nMOS transistor when positively biased. Generally all of the VT degradation due to PBTI may be recovered over time during usage.
TDDB is a failure mechanism that occurs when the gate oxide breaks down as a result of long-time application of relatively low electric field (as opposite to immediate breakdown, which is caused by strong electric field). The breakdown is caused by formation of a conducting path through the gate oxide to substrate due to electron tunneling current, when MOSFETs are operated close to or beyond their specified operating voltages.
At high voltage, both BTI & TDDB contribute to total degradation. Thus, during device development, the stress voltages that are used for accelerated testing and estimating device lifetimes causes both BTI and TDDB. It is difficult to distinguish how much degradation is due to interface traps (BTI) and how much is due to intrinsic oxide quality (TDDB). As a result, the degradation is overestimated. In order to meet device and product specification based on the pessimistic degradation estimates, product design is more difficult and expensive than would be the case if degradation is more accurately predicted.